1. Field of the Invention
The present invention relates to a charging control device for a storage battery, which controls a charging voltage applied to a storage battery in accordance with at least a state of charge of the storage battery.
2. Description of the Related Art
During conventional storage battery management control for improving fuel efficiency, the state of the storage battery must be monitored to detect overcharging and the like early.
Typical well-known methods of estimating a state of charge include a method of calculating the state of charge from an internal resistance value and an electromotive force, a method of calculating the state of charge using the specific gravity of an electrolyte as an index, and a method of constantly measuring a current value discharged from or charged to the storage battery, and calculating the state of charge by integrating the resultant current measurement value.
However, when the current value is integrated continuously over a long time period, an accumulated error increases such that the state of charge determined by the calculation gradually diverges from the actual storage battery state of charge, and therefore, the state of charge of a lead storage battery cannot be grasped accurately. As a result, overcharging and deep discharge increase, causing the lead storage battery to deteriorate.
In response to this problem, Japanese Unexamined Patent Application Publication 2004-85269, for example, discloses a technique in which an open circuit voltage is estimated from the course of a terminal voltage of a lead storage battery, which is measured within a brief time period following the completion of charging/discharging of the lead storage battery, the state of charge is determined on the basis of the estimated open circuit voltage, and a current state of charge is updated sequentially using the determined state of charge in order to determine the state of charge with a high degree of precision.
However, with the technique disclosed in the above document, a lithium ion storage battery or the like, in which variation in the terminal voltage of the storage battery and variation in the state of charge appear substantially synchronously, is unlikely to be affected by an accumulated error. In a storage battery such as a lead storage battery, on the other hand, in which a delay is likely to occur during convergence of the terminal voltage depending on the polarization, it is extremely difficult to estimate the open circuit voltage within a limited time period. Hence, there is a limit to the precision with which the state of charge can be determined on the basis of the open circuit voltage.